Final answer:
A protein is marked for destruction when it has ubiquitin attached. The tagged protein is then degraded by the proteasome, helping to regulate gene expression by controlling protein lifespan.
Step-by-step explanation:
If a protein has ubiquitin attached to it, it is tagged for destruction and recognized by other proteins to be destroyed. The process involves targeting the protein for degradation by marking it with ubiquitin, which serves as a signal that the protein's lifespan has ended. Tagged proteins are subsequently moved to the proteasome, an organelle that is responsible for degrading proteins. The ubiquitin-proteasome pathway is vital for protein turnover and plays a significant role in controlling gene expression by altering the longevity of proteins.